The alt.usage.english FAQ

"wonk" (notes by Fred Shapiro)

(Word Origins)

The OED defines "wonk" as "a studious or hardworking person".
An article in _Sports Illustrated_, 17 Dec. 1962, explains that
in Harvard slang, there was a tripartite classification of students
into wonks, preppies, and jocks. I believe that this classification
is in fact the origin of each of the three terms. The earliest
citations in the OED for the three terms are dated, respectively,
1962, 1970, and 1963. I have found an occurrence of "wonk" in
_Time_ in 1954; an occurrence of "preppie" in the _Cambridge Review_
in 1956; and an occurrence of "jock" in the _Harvard Crimson_ in
1958. In all three instances the context is a Harvard one. (But
Esther Vail recalls: "'jocks'; we called them that at Syracuse
Univ. as early as 1948".)
"Wonk" is said to derive from the word "know" spelled backwards,
but this is not certain. Other suggested origins are the adjective
"wonky" = "weak, shaky", and "wanker" = "masturbator". "Preppy"
comes from "preparatory school". "Jock" (attested from 1922 in the
sense "athletic supporter") comes from "jockstrap", from "jock" =
"penis", from the male name Jack.